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With the arrest of three more suspects, French authorities are widening their investigation into the Bastille Day truck attack that killed 84 people in this French Riviera city to include potential accomplices of the man Isis described as one of its warriors.

French authorities arrested an Albanian man and woman they believed supplied weapons to the 31-year-old attacker, Mohamed Lahouaiej Bouhlel. Authorities also said Bouhlel, who has been described by family and friends as a troubled loner, had sent a text message saying "bring more weapons" to another suspected accomplice and had visited the scene of the attack days before the carnage.

Authorities announced that they had detained the man believed to be the recipient of the text message. Seven others had been detained earlier in connection with Friday's attack. They included Bouhlel's ex-wife, but she was released from custody.

Bouhlel, 31, was a "soldier of the Islamic State," the militant group declared. French authorities said the Tunisian-born attacker was inspired by terrorist organisations.

Twelve victims have not been identified. A 6-month-old child was among 18 people still in critical condition in the hospital. The number of wounded rose to 256, according to the French prosecutor's office, and 85 are still hospitalised.

Bouhlel's text message was sent roughly half an hour before the attack on the landmark Promenade des Anglais, where tens of thousands had gathered to watch a Bastille Day fireworks display. The phone Bouhlel used, police said, was found inside the rented 19-tonne refrigerated truck after he mowed down people for more than a kilometre and was killed by police.

French authorities provided details that suggested the attack was carefully premeditated. Two days before the violence, security-camera footage showed Bouhlel driving the truck on the route, apparently staking it out, Jean-Michel Decugis, head of the judicial police department, told local media. Bouhlel also emptied his bank account, according to one suspect. He was supposed to have returned the truck the day before the attack.

Investigators are also continuing to look into Bouhlel's possible path to radicalisation. It remains unclear whether Isis orchestrated the attack, inspired Bouhlel and was taking responsibility, or was seeking publicity despite having no direct influence.